1. Field of the Invention
The present invention pertains generally to piezoelectric transducers and, particularly, to a method and apparatus for detecting and identifying resonance characteristics of a piezo-electric transducer under test.
2. Description of the Related Art
For the purposes of this application, what is primarily meant by the term "piezo-electric transducers" is piezo-electric pressure sensors, for example, crystal pressure sensors, even though principles of the invention, may also apply to such other transducers such as accelerometers, microphones, hydrophones, piezo-electric final control elements, ultrasound transmitters, loud speakers, elements for typewriter keyboards, etc. The general term "oscillatory properties" is intended to cover the somewhat clearer term "resonance characteristics."
A piezo-electric transducer has oscillatory properties which are dependent upon the structure and nature of the transducer, whereby this dependency is so pronounced that influences of, for example, manufacturing tolerances may be recognized in a resonance characteristic given appropriate measurement and identification of such characteristic. Based upon a deviation of the resonance characteristic of a specific transducer from a reference characteristic prescribable for a series of such transducers, a quality control system can easily be set up for recognition of faulty transducers to thus separate them for further, more precise inspection. Furthermore, knowledge of a resonance characteristic is of great significance for avoiding measuring errors, since, resonance step-up occurs given an excitation of a transducer in the proximity of one of its resonant frequencies and the sensitivity, or, relationship of output signal amplitude to input signal amplitude of a transducer, deviates from an otherwise constant value at this frequency. Changes in the transducer due to, for example, aging, wear or damage can also be noticed in the resonance characteristics. Therefore, information of a type of change can be obtained from a comparison of a current measurement of the transducer resonance characteristics to earlier, stored measurements. As an example, high dynamic transducers may be developed, whereby the resonance characteristic offers a good basis for the evaluation of improvements in the suppression of undesired, low frequency resonances.
However, the resonance characteristics of transducers are determined not only by the transducer itself but also by its acoustically coupled environment. The degree of the coupling, the acoustical properties and the oscillatory modes of the environment are expressed in the measured resonance characteristic and, accordingly, can be recognized from a comparison of same with the resonance characteristic of a free, acoustically uncoupled transducer. Similarly, the, frequently very pronounced, influence of a coupling medium, for example, air or oil, can also be identified from a resonance characteristic.
Measuring methods or apparatus for the identification of oscillatory properties of a piezo-electric transducer, as described above, are known. Examples of such methods or apparatus are disclosed in Austrian patent 369,549. The basic idea is that first, an electronics insert, which can be denoted as a resonance detector module, is supplied with an input signal of a variably prescribable frequency via a frequency generator or synthesizer, and then, an electrical response appearing as a consequence of the piezo-electric properties of the transducer under test connected to this module is identified relative to a reference signal also based upon the input signal. Two subsignals are generated from the input signal, one being independent of the transducer under test and the other being dependent upon the electro-mechanical properties, actually determined at the time of measurement, of the connected transducer. For example, the sub-signals may be generated by using a voltage divider. The transducer under test is incorporated at a suitable location so that it influences one sub-signal, changing its characteristics. Insofar as the two sub-signals are essentially in phase, a difference output signal characterizing the reaction can be generated by a difference or subtraction function. Insofar as the two sub-signals are essentially 180.degree. out of phase, a difference output signal characterizing the reaction of the transducer can be generated by a summing function.
Such measuring arrangements or apparatus are also used for testing or identification of the resonance characteristics of quartz resonators, which are presently being employed to an increasing extent, but which, however, differ quite considerably from what are referred to as piezo-electric transducers in various aspects. The frequencies at which piezo-electric transducers are operated usually lie in the range from 0 to an upper limit of about 400 or 500 kilohertz. In comparison, quartz resonators are usually operated at frequencies above 500 kilohertz. Furthermore, quartz resonators couple relatively far better to a quartz resonator test circuit than do piezo-electric transducers to a piezo-electric transducer test circuit because of differences in the electro-static capacitances of the two devices.
Piezo-electric transducers usually have great inert masses due to transmission members and the like but have only a slight active piezo-electric volume. The sum of the internal capacitance and cable capacitance for typical transducers lies on the order of perhaps 30 to 500 pF. In contrast, capacitance variations measured during the evaluation of the transducer, in the case of a capacitative resistance or bridge arrangement, lies on the order of magnitude of perhaps 10 to 500 fF, i.e., having a capacitance lower by about a factor of 1,000. The evaluation of the small capacitance variations makes particularly high demands of the measuring method or apparatus employed and as such can only be met to a limited degree with the prior art arrangements, given a reasonable outlay of circuits.
Thus, it is highly desirable to have a method and/or apparatus which can be used to identify the oscillatory properties of a piezo-electric transducer which does not require an unreasonable outlay of circuits, in spite of the unfavorable coupling characteristics of such transducers.